1. Field of the Invention
This invention relates to an information recording system using magnetic recording disk units. More particularly, the invention relates to an information recording system which uses a plurality of magnetic recording disk units as multistage storage units.
2. Background Art
Many information recording systems have heretofore been known which employ a plurality of magnetic recording disk units (hereinafter referred to as disk units) such as hard disk drives, like the ones for expanding storage capacity by using a plurality of disk units, or the ones for duplicating information having the same contents by storing them in two different disk units.
FIG. 11 is a block diagram schematically illustrating the constitution of an information recording system by using a conventional disk unit.
An information recording system 1 is connected to a user unit 2, and a control unit 11 writes record information into a disk unit 21 in a storage unit 12 in response to a write request WR received, or reads the record information from the disk unit 21 in response to a read request RD received.
When, for example, the record information is to be written into the disk unit 21, the control unit 11 refers to write control information included in the received information such as the write request WR, and the record information is written into the disk unit 21 according to a form desired by a source of the write request (user unit 2). Further, when the record information is to be read from the disk unit 21, the control unit 11 refers to read control information included in the received information such as the read request RD, and the record information desired by a source of the read request (user unit 2) is read from the disk unit 21 in the storage unit 12 and sent to the source of the read request.
A disk unit, in general, reads and writes information at slower speed than a semiconductor memory. Therefore, a modern disk unit is provided with a semiconductor cache memory at an input/output interface thereof, and holds record information in the cache memory up to an optimum timing, enabling the disk unit to be continuously used for writing or reading the information as much as possible or enabling the information to be read or written at high speed by decreasing a wait time on the side of the disk unit.
There has also been known a system utilizing a plurality of disk units called RAID (redundant array of inexpensive disks or redundant array of independent disks). The RAID has two objects. One object is to decrease loss of record information in case the disk unit has failed by redundantly recording the information in a manner of being dispersed in a plurality of disk units. Another object is to increase the access speed by recording the record information in a dispersed manner. The access speed in this case includes both meanings of increasing the number of bytes that are read or written in a unit time and shortening the response speed.
When, for example, the RAID uses N disk units, the record information is, first, divided into N, the divided record information is recorded into the first to N-th disk units at one time in a divided manner to shorten the recording time. In this case, the N disk units are all so treated as to possess the same function, and recording is effected in rotation by matching the time series of the record information with the order of the first to N-th disk units. By using the RAID, therefore, the access speed can, in principle, be increased to N times as fast.
When the disk unit is used for extended periods of time, in general, the access speed for writing and reading becomes slower than that of when the disk unit is first used. This is because when used for extended periods of time, available recording areas in the disk unit becomes scattered, and an extra seeking time is required for moving a magnetic recording/reproducing head.
FIG. 12 is a diagram illustrating discontiguous recording areas of a disk.
The disk unit 21 that rotates in the direction of an arrow B has a plurality of recording tracks formed in concentric, the tracks being denoted by TR0, TR1, TR2, TR3, - - - , TR7 successively from the outer peripheral side toward the inner peripheral side. The track TR0 of the outermost periphery is a region used by, for example, an operating system (OS) for managing the record information such as files or the like. The next track TR1 is a region which, in the case of, for example, MS-DOS® (or WINDOWS®), is used by a list (file allocation table: FAT) for making the record information such as files or directories corresponding with physical arrangements (e.g., sectors) in the disk. The track TR2 is a region which, in the case of, for example, MS-DOS® (or WINDOWS®), is used by a directory that indicates a file name, attribute, latest date of updating, file size and hierarchical structure. Therefore, the tracks TR0 to TR2 are used for management of the record information. Further, when the OS is not MS-DOS® but is, for example, LINUX of a UNIX® family, a file system called i-node system is used. In this case, some tracks are also similarly used for the management.
The tracks TR3 up to TR7 are data areas, and record information such as files are written therein. A recording area in each track is divided into the smallest physical access units called sectors. Several sectors constitute a cluster (CL), and one file is formed by one or a plurality of clusters (CLs).
A file can be continuously written in the consecutive clusters in, for example, track TR3 at the initial stage of use of the disk unit 21, but is, however, in FIG. 12, written in a cluster CL1 in the track TR3, in a cluster CL2 in the track TR5, in a cluster CL3 in the track TR4, and in a cluster CL4 in the track TR6. Thus, when clusters of a file are discretely written on the disk unit 21, an extra seeking time is needed for moving the magnetic recording/reproducing head as compared with the time when the clusters are continuously written on the same track.
The problem of extra seeking time that is required as the disk unit is used for an extended period of time cannot be solved by the use of the above semiconductor cache memory or by the system which handles plural disk units such as RAID.